Recent developments in telecommunication and semiconductor technologies facilitate the transfer of growing amounts of information over wireless networks. One of the methods for transmitting large amounts of information over wireless medium is known as Orthogonal Frequency Division Multiples (OFDM). In this method multiple subcarriers are used simultaneously to convey wideband (and even ultra wide band) symbols.
Some prior art methods for performing channel estimation were based upon transmitting pilot symbols that were conveyed over subcarriers (pilot subcarriers) that were evenly spaced in the frequency domain. The need to use evenly spaced pilot subcarriers is discussed in “Pilot tone selection for channel estimation in a mobile OFDM system”, R. Negi and J. Cioffi, IEEE Transactions on Consumer Electronics, Vol. 44, No. 3, August 1998.
The use of evenly spaced pilot subcarriers allows to apply the ESPRIT algorithm for estimating the locations (delays) of the taps of the channel impulse response. The ESPRIT algorithm was initially developed for finding oil wells (see: “ESPRIT—Estimation of signal parameters via rotational invariance techniques”, R. Roy and T. Kailath, IEEE Transactions on Acoustics, Speech, and Signal Processing, Vol. 37, No. 7, July 1989).
The implementation of the ESPRIT algorithm to channel estimation is discussed in “Fast multipath delay estimation in OFDM systems using frequency swept pilots”, M. R. Ragahavendra, S. Bhashyam and K. Giridhar, 6th International OFDM-workshop, Dresden. All these mentioned above articles and incorporated herein by reference.
There are various versions of OFDMA schemes. An evenly spaced OFDMA scheme allocated pilot subcarriers that are evenly spaced from each other to a certain user. A non-evenly spaced OFDMA allocation scheme allocates pilot subcarriers that are not necessarily evenly spaced in the frequency domain. Typically, there is a much higher probability that these pilot subcarriers are not evenly spaced.
U.S. patent application 2005/0163257 of Keerthi, which is incorporated herein by reference, describes an evenly spaced OFDMA scheme compliant channel estimation method.
Some transmission schemes, such as non-evenly spaced OFDMA schemes, dynamically allocate transmission resources to one or more users. These transmission schemes allocate multiple pilot subcarriers and multiple information conveying subcarriers to multiple users simultaneously. The distance between the different pilot subcarriers allocated to a certain user are usually not equal. Thus, according to common practice the ESPRIT algorithm can not be applied in non-evenly spaced OFDMA multiple users systems.
A typical non-evenly spaced OFDMA system transmits wideband symbols that are conveyed over 2048 subcarriers, out of which 1680 subcarriers convey information or pilot information, as well as 368 guard subcarriers that do not convey information. Each subcarrier out of the 1680 subcarriers is also referred to as a useful subcarrier.
Three consecutive OFDMA symbols form a slot. Thus, a slot includes the subcarriers that are transmitted during three consecutive symbols.
The useful subcarriers of a slot are arranged in subcarrier tiles. A tile, such as subcarrier tile 10 of FIG. 1, includes four useful subcarriers at each of the three timeslots. The subcarrier tile 10 includes eight information conveying subcarriers 11 and four pilot subcarriers 12-18. The pilot subcarriers 12, 14 16 and 18 are located at the four corners of the subcarrier tile 10. The difference (in the frequency domain) between pilot subcarriers 12 and 14 equals the difference between pilot subcarriers 16 and 18.
FIG. 2 illustrates multiple subcarriers tiles 10-60 that are allocated to a single subchannel.
Each user in the multiple user environment can utilize one or more subchannel. Each subchannel includes one set of six different subcarrier tiles, such as subcarrier tiles 10-60.
Subcarrier tile 20 includes pilot subcarriers 22, 24 26 and 28, subcarrier tile 30 includes pilot subcarriers 32, 34 36 and 38, subcarrier tile 40 includes pilot subcarriers 42, 44 46 and 48, subcarrier tile 50 includes pilot subcarriers 52, 54 56 and 58, and subcarrier tile 60 includes pilot subcarriers 62, 64 66 and 68. These pilot subcarriers are located at the corner of each subcarrier tile.
Each subcarrier tile (10-60) out of the set is randomly selected from a group (91-96) of subcarrier tiles. Each group 91-96 of subcarrier tiles includes seventy consecutive subcarrier tiles.
The difference (in the frequency domain) between different subcarrier tiles that belong to the same subchannel is not equal. More exactly, the probability that the difference is the same is very low.
There is a need to provide an efficient channel estimation method and a receiver that use pilot subcarriers that are not necessarily evenly spaced in the frequency domain.